The new estimate, published in today's Morbidity and Mortality Weekly Report, is based on reports of HIV cases from 40 states and cases of AIDS from 10 states and the District of Columbia in 2006. It is about the same as the previous estimate from 2003, which calculated that between 1 million and 1.2 million people were living with HIV.

But the 2003 estimate may have been an overstatement, Centers for Disease Control and Prevention (CDC) epidemiologists write. This time, they worked with a refined statistical method that includes more reliable counts of HIV infections from more states. Prior to April of this year, not all states tracked HIV the same way, making it difficult for epidemiologists to get a handle on its national impact.

Were the new method applied to 2003, the number of people living with HIV in the U.S. that year would have been lower, about 994,000 people, the CDC scientists write today. That means that since 2003, HIV prevalence has increased by 11 percent, or 112,000 people.

A report this year by UNAIDS, the United Nations agency that tracks the disease around the world, said that an estimated 1.2 million people were living with HIV in the U.S.

Separate, improved epidemiology has shown a sobering picture of HIV before. In August, the CDC reported that it had undercounted annual, new HIV infections by 40 percent. For years, authorities said 40,000 people were infected annually. In fact, that number swung between 49,000 in the early '90s to 58,000 in the late '90s. Today, an estimated 55,000 people are infected each year.

(Image of HIV-1 budding from cultured lymphocyte/CDC)

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